(Not Applicable)
(Not Applicable)
This invention relates to antennas to receive signals from Global Positioning System (GPS) satellites and, more specifically to antennas arranged for high-angle reception for differential GPS applications.
Antenna systems providing a circular polarization characteristic in all directions horizontally and upward from the horizon, with a sharp cut-off characteristic below the horizon are described in U.S. Pat. No. 5,534,882, issued to A. R. Lopez on Jul. 9, 1996. Antennas having such characteristics are particularly suited to reception of signals from GPS satellites.
As described in that patent, application of the GPS for aircraft precision approach and landing guidance is subject to various local and other errors limiting accuracy. Implementation of Differential GPS (DGPS) can provide local corrections to improve accuracy at one or more airports in a localized geographical area. A DGPS ground installation provides corrections for errors, such as ionospheric, tropospheric and satellite clock and ephemeris errors, effective for local use. The ground station may use one or more GPS reception antennas having suitable antenna pattern characteristics. Of particular significance is the desirability of antennas having the characteristic of a unitary phase center of accurately determined position, to permit precision determinations of phase of received signals and avoid introduction of phase discrepancies. Antenna systems having the desired characteristics are described and illustrated in U.S. Pat. No. 5,534,882, which is hereby incorporated herein by reference.
For such applications, antennas utilizing a stack of individually-excited progressive-phase-omnidirectional elements are described in U.S. Pat. No. 6,201,510, issued to A. R. Lopez, R. J. Kumpfbeck and E. M. Newman on Mar. 13, 2001. Elements as described therein include self-contained four-dipole elements which are employed in stacked configuration to provide omnidirectional coverage from the zenith (90xc2x0 elevation) to the horizon (oxc2x0) or from a high elevation angle to the horizon, with a sharp pattern cut off below the horizon. U.S. Pat. No. 6,201,510 is hereby incorporated herein by reference.
In some applications, it may be desirable to employ a set of two antennas, each providing omnidirectional coverage (in azimuth) and the antennas providing complementary coverage in elevation. For example, an antenna pursuant to U.S. Pat. No. 6,201,510 may be designed to provide omnidirectional coverage from the horizon to 55xc2x0 elevation. If available, a second high-angle omnidirectional antenna of appropriate design and performance could be used to provide complementary elevation coverage from 55xc2x0 elevation to the zenith. Used together, such antennas would provide horizon to zenith coverage for omnidirectional reception of GPS signals for DGPS applications. Available antennas (for example, conventional choke-ring antennas as discussed below) have been subject to limitations in areas such as performance, size, cost or reliability.
Objects of the present invention are to provide new and improved antennas, including antennas usable for high-angle reception for DGPS applications and antennas having one or more of the following characteristics and advantages:
omnidirectional coverage from a selected elevation angle to zenith;
high-angle elevation coverage usable in combination with an antenna providing lower elevation coverage;
progressive-phase-omnidirectional radiation pattern;
inclusion of an energy absorber configuration around a plurality of radiating elements to determine lower elevation radiation characteristics;
inclusion of a combination of a cylindrical reflective structure and a cylindrical energy absorber configuration;
inclusion of a serrated-edge cylindrical energy absorber; and
inclusion of an energy absorber utilizing material having radiation absorption properties.
In accordance with the invention, an antenna, having a progressive-phase-omnidirectional excitation network and an absorber configuration, includes first, second, third and fourth dipoles successively spaced around a vertical axis and a signal port. A progressive-phase-omnidirectional (PPO) excitation network, coupled between the signal port and the four dipoles, includes
(a) a first quadrature coupler coupled to the signal port and coupled between the first and second dipoles to provide first dipole excitation of a first phase and to provide second dipole excitation of a quadrature phase, and
(b) a second quadrature coupler coupled to the signal port and coupled between the third and fourth dipoles to provide third dipole excitation of a phase differing by 180 degrees from the first phase and to provide fourth dipole excitation of a quadrature phase relative to the third dipole.
The antenna includes a ground plane section with a reflective surface positioned below the four dipoles, a cylindrical structure coupled to the ground plane section, having a reflective surface, and extending around the four dipoles, and an absorber configuration having a serrated upper-edge portion extending around the four dipoles above the cylindrical structure and having radiation absorption properties.
Also in accordance with the invention, an antenna, with reduced low-angle reception, includes a plurality of antenna elements positioned around a vertical axis and arranged to provide an omnidirectional antenna pattern, a ground plane section with a reflective surface positioned below the antenna elements, a cylindrical structure extending above the ground plane section, having a reflective surface, and extending around the antenna elements, and an absorber configuration extending around the antenna elements above the cylindrical structure and having radiation absorption properties.